From: Eric Benjamin <>
>
>
> I must start out by confessing that I've never used or built a parabolic =
microphone. In fact I've never even seen one. But all of this discussion =
has piqued my interest.
You might want to borrow a Telinga Pro V and it's associated DATStereo
mic element and get out and do some nature recording. Then try and
relate that to theory.
> In order to achieve the theoretical gain, the actual recording circumstan=
ces must result in the waves from various directions arriving at the focus =
with phase errors conforming precisely to the expectation of theory. Put a=
nother way, everything must work exactly right, or the gain will be less th=
an predicted by theory. This can be seen in Wahlstrom's figures 12, 13, an=
d 14. The measured gain is always below the theoretical gain with the exce=
ption of one data point in figure 14.
The theory is ok for thinking about designing a new parabolic mic, but
then you throw all those numbers out and start over. Learning what the
mic actually does in real recording situations. From that point on
theory just gets in the way, you won't get nature to conform to
anything, you live with what it does. The art of using a parabolic is
gained out recording nature with one.
> If follows that Errors of the following types may occur in practice:
>
> Focal length error
> Non conformance to exact paraboloid
> Aiming error
> Violation of the plane-wave assumption
>
> Focal length error: If the microphone is not located precisely at the foc=
al point, then it will be progressively further away from the point where t=
he high-frequency maximum is, and the high-frequency output will be less th=
an the theoretical optimum. This is shown quite clearly in Wahlstrom's fig=
ures 9, 10, and 11. One practical consideration is that the designer might=
like to use a tripod arrangement to support the microphone capsule from th=
e rim of the dish, but for the case where =E1 =3D l this results in a non-r=
igid support system
The Telinga uses a very sturdy central support. Focal length is adjusted
by ear, and if particular can be adjusted to compensate for the exact
distance to your subject this way. Once adjusted and clamped it won't
change. Adjustments made by ear in this way are not errors, they are
deliberate. I currently don't adjust my Telinga, but keep it set about
right for 100 - 200 feet. That does not compromise by much even if used
for distances up to a mile or so. In fact it's hard to find a site
regular enough to precisely focus it.
> Non-conformance to an exact paraboloid results in waves from one directio=
n arriving with positive or negative phase shifts relative to other directi=
ons of incidence. Variations of 1/2 wavelength, which is only about 8.6 mm=
at 20 kHz, will result in complete cancellation. Not only can there be er=
rors due to manufacturing tolerance, but for flexible paraboloids it is ext=
remely unrealistic to expect that they will snap back to exactly their orig=
inal shape. Assuming that the errors are not systematic, the result is mer=
ely a failure for the gain to continue to increase at higher frequencies.
8.6 mm, nearly a cm would be quite a error in a dish the size of the
Telinga. And the required error as frequency drops get's bigger fast. We
don't record at 20kHz in practice, that's just the limit of the range.
At actual frequencies of interest the shape error does not appear to
cause much problem. And the Telinga is a 1mm thick polycarbonate
reflector, easily deformed, so it's usually off by a little bit. A
greater amount if it's recently been rolled up.
> A side implication is that a spherical reflector may work nearly as well =
in practice, and be easier to fabricate.
In field experience a spherical reflector does not work near as well. At
least my field experience.
> Aiming error when measuring (or using) the microphone will result in a ro=
lled-off frequency response. Because the polar pattern of a parabolic micr=
ophone becomes progressively narrower with increasing frequency,
Aiming is not a error, but a tool. I often put things off axis to
control their input.
> A violation of the plane wave assumption will occur if the source of the =
sound is not located at infinity. Again, in order to achieve the theoretic=
al gain from the paraboloid reflector it is necessary that the planarity of=
the wavefront not be in error by more than about 1/4 wavelength. Note tha=
t this effect increases with the size of the dish. A very large paraboloid=
is not appropriate for recording near sources. Assuming that the dish is =
0.5 meters in diameter, and that the distance is 10 m.
>
> So for a dish of diameter 0.5 meters, 5 meters from the source, the "heig=
ht" of the wavefront entering the dish is about 1.2 cm. This will result i=
n cancellation for a source at approximately 14 kHz.
A plane wave? You make a joke. For actual field recording anyway. Make
something that works perfectly only with plane waves and forget it for
nature recording. For a single call you are working on entirely
unpredictable direction, intensity and phase. You will have many
variations of each to record.
> In looking at Wahlstrom's analysis in his appendices, I see that the figu=
res do not precisely portray the shape of the gain curve. Using Wahlstrom'=
s analysis and the result in his Equation 14, I have calculated the theoret=
ical gain curve for a parabolic microphone of dimensions the same as that o=
f the Telinga microphone. The horizontal axis thus takes on the dimensions=
of Hz. The principle thing that can be seen that is not visible in the fi=
gures in Wahlstrom's paper is that the oscillating part of the sound field =
affects the response up to high frequencies. The implication of this is th=
at the sound is dispersed in time, and indeed this would be expected given =
that there is both a direct sound and focused sound component to the microp=
hone signal.
>
> If there is interest on the part of the group, I could expand upon this i=
n more detail, and put the results into the files section of the Nature Rec=
ordist group pages.
One thing you do not seem to understand is that the direct sound will be
very much reduced in intensity compared to the reflected sound.
Interference from the direct sound is some problem at low frequencies
particularly for parabolas of some designs, but it's effect is extremely
minor at high frequencies.
Walt
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